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Watershed assessment of the Cottonwood and Whitewater Watersheds

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Watershed assessment of the Cottonwood and Whitewater Watersheds


Published 2005
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"Prepared for: Malta Field Office, Bureau of Land Management Malta, Montana."

Includes bibliographic references [p. 54-57]

The Cottonwood and Whitewater 4th-code watersheds are located in Blaine and Phillips County, near the boundary of the Northern Great Plains prairie pothole region in north-central Montana. In 2003, the Montana Heritage Program completed an assessment of the Whitewater watershed (Crowe and Kudray 2003). Under agreement with the BLM, that work was extended to the Cottonwood watershed, and the original study data was reanalyzed and updated so that 5th-code watersheds within the two larger 4th-code watersheds (plus portions of the Middle Milk) could be compared. The study area encompasses 1,139,021 acres, of which 4.7% (53,488 acres) are wetlands. Uplands comprise almost 95% of the watershed (1,085,533 acres). There are 1,286 miles of perennial and intermittent streams. Slightly over 40% of the study area is publicly owned or managed, with 78.6% of public land under BLM administration. Most of the land area is grassland, and both public and private grasslands are used primarily for cattle grazing. Approximately 35% of the study area is in agricultural use (hay, small grains, row crops, or fallow). Twenty-seven percent of the land within 100 meters of lentic wetlands and 9% of the land surrounding perennial and intermittent streams is in agriculture. Across the watershed, 46.1% of lacustrine wetland acres and 7.9% of palustrine wetland acres have been hydrologically modified. Slightly over 6% of palustrine wetlands in the study area are impounded. Our methodology included both broad-scale GIS and fine-scale field assessments. The GIS analysis examined underlying diversity, measured current conditions, and evaluated potential threats. Field sampling included proper functioning condition determinations, intensive riparian assessments, and aquatic community inventories. In our GIS assessment, we characterized underlying diversity within 5th code watersheds on the basis of soilbased ecosites, topography, and wetland type/ distribution. When the three measures of diversity were combined into a Composite Diversity Index, Woody Island Coulee ranked highest overall, and Buckley Creek lowest. To assess wetland and watershed condition, we gathered and analyzed data on land cover and land use, natural vegetation communities, land stewardship, water diversion, and wetland/riparian disturbance. We calculated a Composite Wetland Condition Index from seven sub-indices. Whitewater watershed had the highest condition ranking, and Murray Coulee the lowest. We then used a Composite Wetland Threat Index to evaluate ongoing threats from grazing and agriculture, and to assess the potential threats from agricultural conversion and protracted drought. For most of the watersheds, grazing and drought were the major threats. Murray Coulee and Sneider Coulee watersheds were the most threatened of the 5th-code HUCs. Several key facts emerged from the GIS data analysis: 1. Based on cadastral data and allotment boundaries, between 81% and 98% of the land in natural cover is grazed. 2. Comparisons between expected natural communities and current land cover indicate that the greatest loss of community type has occurred in shrub/evergreen communities. 3. More than a third of the wetlands across the study area have some direct disturbance as a result of hydrological alteration or stockwatering activities. 4. Surface water is a highly manipulated resource throughout the study area, and free-flowing channels are probably rare. In the Whitewater watershed, for example, there are over 27 dams and diversions per mile of perennial and intermittent stream. 5. Fifty percent or more of the streams in every watershed except Black Coulee are within 50 meters of a road. As part of the fine-scale assessment, we surveyed 161 potholes and wetlands across the study area. PFC assessments were done at 97 sites (some sites had more than one pothole). Of the 97 sites surveyed, 30, or 31%, were considered to be functioning at risk. We also carried out intensive riparian assessments at 17 sites and calculated an overall Floristic Quality Index, the percentage of non-native species, the total percentage of species that are tolerant to disturbance, and the percentage of species that are intolerant to disturbance. Almost all sites exhibited a high percentage of disturbance tolerant species, with values as high as 0.0% to 69.4% for woody species, and 68.2% for herbaceous species. Non-native herbaceous species were common throughout the riparian area. During our aquatic condition inventories, we found that most of the unconnected streams were dry, and many 2nd order streams contained no water or only interrupted pools. The mainstem of Cottonwood Creek is severely incised and continually downgrading its channel, and contains few of the expected fish species for a stream this size. Woody Island Coulee contained the most intact fish community, and has many stream reaches with high biological integrity. Assiniboine Creek (in Stinky Creek watershed) had a full complement of expected species. We did not find clear relationships between the broad-scale and fine-scale assessments. Broadscale assessments look at impacts, i.e. the activities and events that change natural conditions, while fine-scale assessments examine the results of those impacts. Impacts may occur at a significance distance from their effects. Localized impacts may also override watershed-level ones. In our visits to wetlands in the study area, we observed that the most significant effects on plant community composition and proper functioning condition corresponded to local impacts of grazing and/or hydrologic alteration. The value of watershed-level assessments lies in identifying areas where impacts are currently occurring, rather than merely seeking out effects that have already occurred. Based on both levels of assessment, we identified several management opportunities that would support wetland and watershed health: 1. Placement of stockwatering tanks, nutrient feeders and salt blocks in places with a low concentration of wetlands; exploration of rotational grazing to protect breeding waterfowl in spring and to limit trampling of potholes in late summer; increased range condition monitoring; and protection of high quality wetlands with physical barriers. 2. Avoiding direct encroachments by oil and gas pipelines in wetlands, and planning associated roads to minimize impacts from dust, traffic, and erosion. 3. Continuing to monitor for noxious weeds. The study area is unusually free of noxious weeds. 4. Management of lands around Woody Island Coulee to protect the aquatic resource. In general, the study area has not suffered the same level of impacts as many parts of the Northern Great Plains Prairie Pothole Region, and a high percentage of its wetlands are still functioning and intact. However, increased oil and gas development, drought, overgrazing and noxious weeds all represent significant threats

Agreement Number :


Volume 2005
Publisher Helena, MT : Montana Natural Heritage Program
Pages 97
Language English
Call number 577.68
Book contributor Montana State Library
Contributor usage rights See terms
Collection MontanaStateLibrary; americana

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